I have been there!!!
I did the same thing when I first started rolling, it was really depressing when everyone else made it look so easy.
Everyone kept telling me to make sure I was round............I WAS TRYING!! but after I hurt my shoulder, I was tense and then it just hurt more.

disclaimer:
I am still a newb so my suggestions are in no way scientific or borne from years of experience, but they worked for me.

I take ibuprofen 800 mg about 45 mins before class, (kills some of the pain and reduces inflamation) and make sure my weight is on my front leg and then kick my back leg as I go over. I also still use two hands to try to get rounder. Try not to go too slow, it makes it rougher.
Just don't quit trying, you will be able to roll again eventually. I could.